The invention relates to a cholesteric polarizer comprising an optically active layer of a polymer material having a cholesteric order, the material being oriented in such a manner that the axis of the molecular helix extends transversely to the layer. The invention also relates to methods of manufacturing such polarizers. The invention further relates to a lighting device comprising a socket for an electric light source, a reflector and such a cholesteric polarizer which is preferably provided with a quarter-wave plate.
Polarizers are used to convert unpolarized light into polarized light. Until now so-called "sheet polarizers" have been used for this purpose. When said sheet polarizers are exposed to unpolarized light they transmit one of the two orthogonically linearly polarized components of the light, while the other component is absorbed in the polarizer. Such polarizers have the drawback that under optimum conditions maximally only 50% of the quantity of incident light is converted into polarized light. Thus, this type of polarizers has a relatively low efficiency. Another drawback relates to the absorption of the untransmitted component. This may give rise to considerable heating of the polarizer, which causes undesired changes in the polarization characteristic of the polarizer and, at high intensities of the incident light, can even lead to destruction of the polarizer.
By means of cholesteric polarizers it is possible to very efficiently convert unpolarized light into polarized light. Such polarizers comprise an optically active layer of a cholesteric (i.e. chiral nematic) material. In this type of liquid crystalline material the chiral molecules have a structure such that they spontaneously assume a spiral-like or helical structure. After such a mixture has been provided as a thin, optically active layer between two parallel substrates, said helical structure is oriented in such a manner that the axis of the helix extends transversely to the layer. A better orientation of the helix is obtained if the substrates are provided with so-called orientation layers on the surfaces facing each other. If this type of polarizer is irradiated with a beam of unpolarized light, the part of the light which is "compatible" with the (right-handed or left-handed) direction and pitch of the helix is reflected, while the remainder of the light is transmitted. By means of a mirror, the "compatible" polarization of the reflected light can be reversed, after which said light, which is now "incompatibly" polarized, can again be directed on to the polarizer. In this manner and using this type of polarizer, theoretically, 100% of the incident unpolarized light having a "compatible" wavelength can be convened into circularly polarized light.
Such a cholesteric polarizer is known from an article by Maurer et al., entitled "Polarizing Color Filters Made From Cholesteric LC Silicones", from SID 90 Digests, 1990, pp. 110-113. In this article a description is given of cholesteric polarizers whose optically active layer consists of a polymer material having a cholesteric order on the basis of silicones. This layer is manufactured by orienting a mixture of a chiral silicone monomer and a nematogenic silicone monomer between two substrates of glass, after which they are polymerized to the optically active layer by means of UV light. The ratio between the two types of monomer in the polymer material governs the pitch of the molecular helix and the reflection wavelength (=colour of the reflection) associated therewith. The ratio between the pitch p and the wavelength .lambda. is given by the formula .lambda.=1/2.(n'+n")p, where n' and n" are the extraordinary and the ordinary refractive index, respectively, of the polymer material.
An important drawback of the known cholesteric polarizer is that the bandwidth .DELTA..lambda. of the polarized light is much smaller than the bandwidth of the visible spectrum. This bandwidth is determined by the formula .DELTA..lambda.=.lambda...DELTA.n/n, where .DELTA.n=n'-n" represents the birefringence of the layer and n=(n'+n")/2 represents the average refractive index. The bandwidth in the visible portion of the light spectrum is governed predominantly by the birefringence of the cholesteric material. The possibilities of increasing said birefringence are relatively limited. In practice it has been found that .DELTA.n is smaller than 0.3, so that the associated bandwidth is smaller than 100 nm. In general, the bandwidths have values ranging between 30 and 50 nm. This small bandwidth is problematic for many applications. In practice, polarizers having a bandwidth of at least 100 nm, and preferably 150 nm and more are desired. In particular bandwidths which cover an important portion of the visible spectrum are very interesting for industrial applications.
In the above-mentioned article this known problem is overcome by the use of polarizers which are built up of a number of optically active layers having different reflection wavelengths. In this manner, a polarizer having a bandwidth of 300 nm can be obtained which covers substantially the entire visible portion of the spectrum. However, this solution has a number of important drawbacks. First, the optical quality of cholesteric polarizers consisting of more than one optically active layer deteriorates rapidly due to errors which are typical of cholesterics. Said errors are, in particular, so-called "focal-conical" disclinations, "Grandjean"-disclinations and a loss of planar molecular order. Second, the thickness of such a composite polarizer causes problems. As the thickness of the individual layers must minimally be 6 microns, such composite polarizers have a minimum thickness of approximately 20 microns. At such thicknesses of the optical layer, the polarizer becomes excessively dependent on the viewing angle.